It has been considered that use of a power supply unit which can generate a current pulse with a high peak and with a narrow pulse width as a power supply unit for machining is the best way to improve a machining speed of electric discharge machining. For this reason, an FET is generally used as a switching device, and a plurality of FETs are connected in parallel to each other to supply a large current to a space between the electrodes by directly turning ON/OFF a current passing therethrough with a switching circuit.
Also, a power supply circuit comprises two units of switching circuit, namely a high impedance circuit which has a small current peak value and only supplies a voltage to a space between electrodes and a low impedance circuit which has a large current peak value, and a uniform discharge current is formed by at first applying a voltage to a space between electrodes with the high impedance circuit and then closing the low impedance circuit for a specified period of time after the generation of electric discharge is detected to flow a desired current therethrough, whereby a machining speed to surface roughness is improved.
FIG. 8 shows one example of a power supply apparatus for an electric discharge machine based on the conventional technology. This power supply apparatus has a high impedance circuit 10 and a low impedance circuit 20, and both of the high impedance circuit 10 and the low impedance circuit 20 are connected to a tool electrode 1 as well as to a workpiece W each as a power supply circuit. Each of the power supply circuits includes floating inductance L due to wiring or the like in the circuit.
The high impedance circuit 10 comprises a DC power supply unit 101, a semiconductor switching device 102, a control unit 103 for controlling ON/OFF of the semiconductor switching device 102, a diode 104, and a resistor 105 for restricting a current in the high impedance circuit 10.
The low impedance circuit 20 comprises a DC power supply unit 201, a semiconductor switching device 202, a control unit 203 for turning ON/OFF the semiconductor switching device 202, a diode 204, and a constant-voltage circuit 205. The constant-voltage circuit 205 comprises a capacitor 206 placed on an input side, a switching device 207 for the constant-voltage circuit, a control unit 208 for controlling ON/OFF of the switching device 207 for the constant-voltage circuit, and a resistor 209.
Next description is made for operations of the power supply apparatus for an electric discharge machine having the configuration as described above with reference to FIGS. 9A to 9G.
When the switching device 102 in the high impedance circuit 10 is tuned ON, a voltage E1 as shown in FIG. 9A is applied to the space between the electrodes. Insulation between the electrodes formed by the tool electrode 1 and the workpiece W is broken down by the voltage E1, and then electric discharge is generated therebetween. The switching device 202 in the low impedance circuit 20 is tuned ON when the generation of electric discharge is detected and a large current is supplied to the space therebetween, whereby electric discharge machining is carried out.
In a state in which the electric discharge has been generated between the electrodes and a discharge current is flowing, the voltage therebetween maintains an arc potential Vg (around 20 V).
FIGS. 9E, 9F and 9G show ON/OFF states of the switching devices 102, 202, 207, respectively.
Herein detailed description is made for a current waveform of the low impedance circuit 20 at the time when the circuit is turned ON or OFF. A circuit from the DC power supply unit 201 to the space between the tool electrode 1 and the workpiece W in the low impedance circuit 20 is a non-resistance circuit not including a resistor or the like excluding the semiconductor switching device 202. Accordingly, when the switching device 202 is ON, if the insulation between the tool electrode 1 and the workpiece W is broken down, a current flows, but the current at that time rises at a slope decided by a voltage E2 of the DC power supply unit 201 and also floating inductance L in the circuit.
Herein, even if it is assumed that the switching device 202 is an ideal one with the turn-OFF time of zero, even when the switching device 202 is changed from ON to OFF, a current can not be reduced to zero even momentarily because of some energy accumulated in the inductance in the line. For this reason, so-called a surge voltage is generated at the both edges of the switching device 202, a current charging the capacitor 206 in the constant-voltage circuit 205 flows through the diode 204, and the voltage of the capacitor 206 increases.
The constant-voltage circuit 205 is constructed so as to control a duty ratio of ON/OFF of the switching device 207 so that the voltage of the capacitor 206 becomes constant, and the energy temporarily accumulated in the capacitor 206 is eventually consumed by the resistor 209.
A current I flowing through the electrodes is expressed by a sum of a current I1 flowing through the switching device 202 and a current I2 flown in the diode 204, which is shown in FIGS. 9B, 9C and 9D, respectively.
A slope of a rise of a current when the switching device 202 is tuned ON is expressed by the following expression: EQU (E2-Vg)/L
and a slope of a fall of the current at the time of turning OFF is expressed as follows: EQU (E3+Vg)/L
wherein designated at the sign E2 is a voltage of the DC power supply unit 201, at E3 a voltage in the constant-voltage circuit 205, at Vg an arc voltage between the electrodes, and at L an inductance in the line.
In summary, after the high impedance circuit 10 is turned ON, electric discharge is generated between the electrodes, and when the switching device 202 in the low impedance circuit 20 is turned ON, a peak current expressed by the following expression flows between the electrodes after an ON-time t1 has passed: EQU Ip=(E1-Vg)t1/L
and machining is performed.
There are some other ones, as a power supply apparatus for an electric discharge machine based on the conventional technology other than the power supply apparatus for an electric discharge machine shown in FIG. 8, disclosed in Japanese Patent Laid-Open Publication No. SHO 49-118097, in Japanese Patent Laid-Open Publication No. HEI 5-84609, and in Japanese Patent Laid-Open Publication No. SHO 63-7225.
The power supply apparatus for an electric discharge machine based on the conventional technology (shown in FIG. 8) has the configuration as described above, so that, to improve a machining speed, it is required to make larger the number of switching devices connected to each other in parallel to generate a pulse with a high current peak value.
However, in electric discharge machining, generally finishing is performed after rough machining is carried out, and in finishing, machining is executed at a pulse with a low machining current value to improve machining precision and machined surface roughness in the finishing. Accordingly, as far as the number of switching devices is concerned, the object to improve a machining speed is contradictory to an object to improve machining precision.
For this reason, it is difficult to achieve both of the improvement in machining speed and that in machining precision in the same power supply apparatus, therefore two types of power supply apparatus have to be used for finishing at a low peak current and for roughing at a high peak current.
Also, in a fine electrode with a wire diameter of .phi. 0.15 mm or less, when a power supply apparatus with a high current peak value is used, a current peak value is too high as a machining condition for roughing so that a wire is cut off and machining can not be executed. Also, in this case, in the low peak power supply apparatus for finishing, a machining speed is too slow to practically be used.
Conventionally, when roughing a workpiece with a fine electrode with a wire dimension of .phi.0.15 mm or less, the above problems have been dealt with a method of making a current peak value smaller by decreasing a supply voltage of the DC power supply unit in the switching circuit or the like.
A current peak value is expressed by the following expression, Ip=(E1-Vg)t1/L, so that, if a supply voltage E is made to 1/3, a peak current in a minimum set time can be suppressed to around 1/3. FIG. 10A shows a current waveform at a normal time when the supply voltage E is used as it is, and FIG. 10B shows a current waveform in a case where the power supply voltage E has been suppressed to 1/3 respectively. It should be noted that, in FIGS. 10A and 10B, tmin indicates a width of a minimum ON time to be set for a switching device.
However, when a supply voltage in the switching circuit is decreased, a rise of a current peak voltage becomes dull, deposition of a wire electrode onto a workpiece is generated, and a machined groove is filled with the deposited materials, which makes it impossible to continue machining. In addition, to decrease a supply voltage in the switching circuit, it is required to have a plurality of DC power supply units, whereby problems in aspects of space-saving and costs become more serious.
In the electric discharge machine disclosed in the Japanese Patent Laid-Open Publication No. SHO 49-118097 or in Japanese Patent Laid-Open Publication No. HEI 5-84609, consumption of a tool electrode is suppressed by gradually increasing the number of switching devices to be ON. But, in each of the electric discharge machine according to those inventions, although some effects can be achieved in an area in which a pulse width is very wide, when it is used in finishing in wire electric discharge machining and in roughing with a fine electrode, largely different from a machining a wire electric discharge machine with a high peak and a small pulse width, none of the problems such as generation of a wire to be cut off or deposition of an electrode material onto a workpiece can be solved.
Also, the electric discharge machine disclosed in Japanese Patent Laid-Open Publication No. SHO 63-7225 has configuration in which a plurality units of switching device in the main power supply circuit are connected in parallel to each other, and the number of parallel circuits is changed by a driving circuit for discretely driving a parallel device group by a control signal from the control unit according to a state of electric discharging between a workpiece and a wire electrode, and machining is executed by a discharge current according to an electric discharging state between the electrodes. In this electric discharge machine, in a case where electric discharging is used in finishing such as a second cut, the number of switching devices to be simultaneously tuned ON is selectively controlled, so that machining can be performed at a current peak value suited for the finishing condition.
However, there is a relation as shown in FIG. 11 between the number of switching devices to be simultaneously tuned ON connected in parallel to each other and a current peak value, so that, even if the number of switching devices to be simultaneously tuned ON is increased, a current peak value is not always increased linearly in accordance with increase of the number of ON-units thereof. Namely, even if the number of the switching devices connected in parallel to each other (the number of units to be simultaneously turned ON) is decreased to 1/10, a current peak value of 1/10 of that when all of the switching devices is simultaneously tuned ON can not always be obtained.
It is well known that there is a nonlinear relation between the number of parallel units and a current peak as described above. Namely, a current peak becomes discrete, as shown in FIG. 12, when control is provided only by increasing or decreasing the number of switching devices (the number of units to be simultaneously turned ON), so that it is difficult to apply the control system to machining conditions in a broad range from machining with a fine wire to roughing with a wire electrode with the thickness of around .phi. 0.2 mm.
Also, as shown in the example of the conventional technology in FIG. 8, a current peak value becomes discrete, when control is provided by controlling an ON time of the switching device, in machining with a fine wire as an electrode, as described above.